HP DeskJet 695 printer

I have a problem with my printer.
it is a DeskJet 695 printer HP
all of a sudden I started having problems with the colors. for example instead of blue I ended up with green color.
most colors are now printed green instead.
I have to admit I didnot clean the printer often.
I thought this might be due to the cartridge being empty, so I bought a new black and a colored one.
But I still have the same problem.
What could that be?